Video: What is Granite? - Definition & Colors

Two places where you might be most likely to encounter granite are kitchens and cemeteries. But the material's importance goes far beyond its uses because it forms the foundation of our continents. Learn all about it in this lesson.

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Two places where you might be most likely to encounter granite are kitchens and cemeteries. But the material's importance goes far beyond its uses because it forms the foundation of our continents. Learn all about it in this lesson.

Definition

Granite is an igneous rock composed of mostly two minerals: quartz and feldspar. It is an intrusive rock, meaning that it crystallized from magma that cooled far below the Earth's surface. Its name is derived from the Latin word 'granum,' which means 'grain,' a reference to the easily-seen minerals in the rock.

Where Is Granite Found?

Much of the earth's continental crust is made of granite, and it forms the cores of the continents. In North America, the landscape surrounding Canada's Hudson Bay and extending south to Minnesota consists of granite bedrock. These rocks are part of the Canadian Shield, the oldest rocks on the continent.

Granite also is found below much of the rest of the middle of the continent. Buried under hundreds of feet of sedimentary rocks and glacier-deposited sediment, you'll find what's called basement rock. Granite can make up much of this foundation of the continents.

In mountain ranges like the Sierra Nevada, Appalachians, and Rocky Mountains, granite is found in huge masses of rock called batholiths which form the roots of the mountains. Half Dome and Pike's Peak are mountains sculpted from granite batholiths.

How Does Granite Form?

Given the abundance of granite, it's not surprising to learn that geologists still have many questions about how it forms. Sure, it comes from molten rock, but just where did all that magma come from? And how far below ground did the magma crystallize?

Probably the most widely accepted idea (at least at the moment) is that granite magma originated from a mechanism called partial melting, in which rocks of a very different composition melt in stages and the initial magma is enriched in the minerals that melt first. But where that happens - whether in the mantle or in the lower lithosphere - remains unclear. Regardless of where the magma formed, it probably migrated upward before collecting in large magma chambers prior to cooling and solidifying.

Mineral Composition

Although the term 'granite' or 'granitic' is sometimes used as a general description for any intrusive rocks that look like granite, the name really applies to a rock with a very specific mineral composition. Granite is composed mostly of two minerals: quartz and orthoclase feldspar (a potassium-rich variety of feldspar). Quartz must make up at least 20% of the rock and orthoclase at least 35%.

If either of those criteria is not met, then the rock is not granite. In fact, those are the only two minerals that have to be in the rock! The remaining rock (up to 45%) can be one or more other minerals, such as plagioclase feldspar (a sodium-rich variety), hornblende, pyroxene, muscovite, or biotite (the last two are kinds of mica).

Texture and Color

There are two obvious physical properties of granite that determine what it looks like: its texture (the size of the individual mineral grains) and its color. The variability in these two properties leads to a wide range of the appearance of granite.

The individual minerals in granite grow into visible grains because the magma cools slowly many miles below the surface. It is the size of the grains of different minerals that imparts the 'speckled' look to the rock.

All granite has what is called coarse-grained texture, meaning that the individual mineral grains are visible to the naked eye. The fancy term for that is 'phaneritic texture.' Under certain conditions, the mineral grains can grow very large. When that happens, the granite is called a pegmatite.

The overall color of granite depends largely on the kind of feldspar in the rock. Potassium-rich feldspar tends to be some shade of red or pinkish-tan, so a lot of granite is similarly red or pink. But if the rock has a lot of sodium-rich feldspar in it, which is typically white or gray in color, the granite will be gray.

Uses for Granite

Aside from countertops and headstones, granite is used as a decorative facing stone, a floor tile, and as a construction material. The rock is massive and very strong, and it was once a commonly-used building stone. Aberdeen, Scotland, has so many buildings made of granite, it is called 'The Granite City.'

Example from the Granite City

Example from the Granite City

Some granite pegmatites contain minerals with high concentrations of rare elements, such as lithium. Granite also typically contains uranium-bearing minerals, although the uranium is not usually present in quantities that are economical to mine. However, many uranium deposits are thought to contain uranium from weathered granite.

Weathering also breaks apart granite into piles of individual mineral grains called grus, which are then swept away into streams where they become sand. Play sand and beach sand, as well as the sand in sandstone, is largely quartz and feldspar grains produced from weathered granite.

Lesson Summary

Granite is a coarse-grained, intrusive igneous rock that composes the cores of the continents and the roots of mountain ranges. Granite contains mostly quartz and orthoclase feldspar; other minerals make up less than half of the rock. The magma that crystallizes into granite likely forms in the upper mantle or lower lithosphere where other rocks partially melt. Granite is used in buildings and some granite is mined for rare elements. It also gives us sand and sandstone.

Pertinent Terminology

Terms

Definitions/Facts

Granite

intrusive igneous rock inside the cores of the continents and the roots of mountains

Intrusive rock

crystallized from magma

Continental crust

contains the granite that forms the cores of the continents

Basement rock

found below much of the middle of the continent; buried under hundreds of feet of sedimentary rocks and glacier-deposited sediment

Batholiths

form the roots of the mountains like Half Dome and Pike's Peak

Partial melting

rocks of a very different composition melt in stages, and the initial magma is enriched in the minerals that melt first

Quartz and orthoclase feldspar

the main minerals in granite

Remaining rock

(45%) could be plagioclase feldspar, hornblende, pyroxene, muscovite, or biotite

Learning Outcomes

When finished studying, find out if you've learned enough about this lesson to:

Summary:

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